In recent years, multi-layer transformers have attracted attention that are thin, small, and lightweight in accordance with rapid advances in the miniaturization of electronic devices. FIG. 13 is a disassembled perspective view of a stacked body of a conventional multi-layer transformer. FIG. 14 is a vertical cross-sectional view along the line XIV-XIV in FIG. 13 after stacking. The description below is based on FIGS. 13 and 14.
A conventional multi-layer transformer 80 comprises primary-winding magnetic sheets 82b and 82d on which primary windings 81a and 81c are formed, secondary-winding magnetic sheets 82c and 82e on which secondary windings 81b and 81d are formed, and magnetic sheets 82a and 82g that hold the magnetic sheets 82b to 82e from both sides.
Furthermore, a magnetic sheet 82f for improving the magnetic saturation characteristic is inserted between the magnetic sheet 82e and magnetic sheet 82g. The magnetic sheets 82a to 82e are provided with through-holes 90, 91, and 92 that connect the primary windings 81a and 81c and through-holes 93, 94, and 95 that connect the secondary windings 81b and 81d. The lower face of the magnetic sheet 82a is provided with primary-winding external electrodes 96 and 97 and secondary-winding external electrodes 98 and 99. The through-holes 90 to 96 are filled with a conductor. The magnetic sheets 82a to 82g are the core of the multi-layer transformer 80.
Further, FIGS. 13 and 14 are schematic diagrams and, therefore, strictly speaking, the number of windings of the primary windings 81a and 81c and secondary windings 81b and 81d and the positions of the through-holes 90 to 96 do not correspond in FIGS. 13 and 14.
On the primary side of the multi-layer transformer 80, the current flows in the order of the external electrode 96, through-hole 92, primary winding 81c, through-hole 91, primary winding 81a, through-hole 90, and then the external electrode 97 or in the reverse order. On the other hand, on the secondary side of the multi-layer transformer 80, the current flows in the order of the external electrode 99, the through-hole 95, the secondary winding 81d, the through-hole 94, the secondary winding 81b, the through-hole 93, and then the external electrode 98 or in the reverse order. The current flowing through the primary windings 81a and 81c produces a magnetic flux 100 (FIG. 14) in the magnetic sheets 82a to 82g. The magnetic flux 100 produces an electromotive force corresponding with the winding ratio in the secondary windings 81b and 81d. The multi-layer transformer 80 operates thus.
Here, supposing that the self-inductance of the primary windings 81a and 81c is L1, the self-inductance of the secondary windings 81b and 81d is L2, the mutual inductance of the primary windings 81a and 81c and the secondary windings 81b and 81d is M, and a magnetic coupling coefficient k is defined by the following equation:k=|M|/√{square root over ( )}(L1·L2) (k≦1)
The magnetic coupling coefficient k is one of the indicators of the transformer function and the larger the magnetic coupling coefficient k, the smaller the leakage magnetic flux (leakage inductance) becomes and, therefore, the power conversion efficiency is high.
Problem to Solved
The multi-layer transformer 80 is mounted on a printed wiring board as an individual component, for example. However, it is becoming more and more difficult for such prior art to respond to demand for further reductions in the size of electronic equipment.
Further, in the multi-layer transformer 80, a magnetic layer (the magnetic sheets 82c to 82e) is formed between the primary windings 81a, 81c and secondary windings 81b, 81d, causing magnetic flux leakage 86 (FIG. 14), and hence it is not possible to obtain a sufficient electromagnetic coupling coefficient k. To solve this problem, a technique (referred to hereafter as a “conventional multi-layer transformer”) has been considered whereby a dielectric layer (not shown) is provided on the primary windings 81a, 81c and secondary windings 81b, 81d by means of screen printing or paste coating such that the magnetic permeability of the magnetic layer is reduced by the substances which diffuse from the dielectric layer.
However, in this conventional multi-layer transformer, conductive substances (Ag particles, for example) may diffuse from the primary windings 81a, 81c and secondary windings 81b, 81d onto the dielectric paste coated on the primary windings 81a, 81c and secondary windings 81b, 81d, leading to a decrease in the insulating property between the primary windings 81a, the primary windings 81c, the secondary windings 81b, and the secondary windings 81d. This is because the paste takes a liquid form due to an organic solvent or the like, for example, and hence substances diffuse easily therefrom.
Moreover, when a dielectric layer is provided in order to reduce magnetic flux leakage, the gap between the primary windings 81a, 81c and secondary windings 81b, 81d corresponds to “the magnetic layer+the dielectric layer”, and therefore widens. As a result, magnetic flux becomes more likely to leak into the gap, causing the electromagnetic coupling coefficient k to decrease. Therefore, with this conventional multi-layer transformer, it is extremely difficult to increase the electromagnetic coupling coefficient k.